The present invention is directed to ink compositions and, more specifically, the present invention relates to conductive inks which have the power or quality of transmitting electrical signals generated by electric field assisted acoustic ink jet printing processes and apparatuses, resulting in controlled ink jettability which in turn improves the edge raggedness of the images. The conductivity of a material can be measured in terms of reciprocal of resistivity, which is the capacity for electrical resistance. The conductivity values of the invention inks expressed as log(pico.mho/cm) and recited herein were measured under melt conditions at 150xc2x0 C. by placing an aluminum electrode in the molten ink and reading the resistivity output on a GenRad 1689 precision RLC Digibridge at a frequency of 1 K.Hz. Conductivity expressed in terms of [log(pico.mho/cm)] is calculated from the reciprocal of resistivity. Generally, the invention inks possess conductivity values in the range of from about 2 to about 8.5 log(picomho/cm), and preferably in the range of from about 6 to about 8.5 log(picomho/cm), with a melting point of between about 60xc2x0 C. and about 150xc2x0 C., and preferably between about 70xc2x0 C. to about 90xc2x0 C., and such inks are especially useful for electric field assisted acoustic ink jet printing with enhanced jettability, processes and apparatuses, reference, for example, U.S. Pat. No. 5,121,141, U.S. Pat. No. 5,111,220, and U.S. Pat. No. 5,371,531, the disclosures of which are totally incorporated herein by reference, including especially acoustic ink processes as illustrated in some of the aforementioned copending applications and patents, such as an acoustic ink printer for printing images on a record medium.
The inks of the present invention in embodiments thereof can be considered a phase change ink, that is, for example, an ink that changes, for example, the crystal structure of the entire ink components so it is a physico-chemical process that is, for example, more physical rather than chemical, from a liquid state to solid state in a suitable period of time, for example, from about 1 to about 100 milliseconds, and preferably in less than about 10, such as from about 2 to about 7 milliseconds; (1) a conductive mixture of a urea or a thiourea compound and an oxyalkylene compound having a melting point of lower than about 120xc2x0 C., and preferably between about 75xc2x0 C. to about 100xc2x0 C., that, for example, can fill the pores of the paper and with a low acoustic loss value of below about 100 dB/mm; (2) a polymeric binder having a melting point of between about 60xc2x0 C. to about 120xc2x0 C., and preferable between about 80xc2x0 C. to about 100xc2x0 C., that can improve the adhesion of ink to paper; (3) a lightfast UV absorber; (4) a lightfast antioxidant; (5) and a colorant such as a dye, a pigment or mixtures thereof.
More specifically, the present invention is directed to phase-change acoustic ink compositions comprised of (1) a conductive mixture of an organic compound such as urea, thiourea, and their derivatives and an oxyalkylene compound such as oxyalkylene bisamides such as N,Nxe2x80x2-propyleneoxy-propyleneoxy-propylene)-bis-stearamide; N, Nxe2x80x2-propyleneoxy-propyleneoxy-propylene oxy-propylene-)-bis-stearamide; poly(alkyleneoxide) alkylates such as N,Nxe2x80x2-(ethyleneoxy-ethyleneoxy-ethyleneoxy-ethyleneoxy-ethylene)-stearate; polyoxa-alkanedioate diester such as distearyl-3,6,9-trioxaundecanedioate, having a melting point of between about 75 and about 100xc2x0 C., and having low acoustic loss and which acoustic loss is below about 100 dB/mm, and preferably in the range of between 25 to about 80 dB/mm; (2) a polymeric binder with, for example, a melting point of between 60 about to about 120xc2x0 C., and preferably between about 80xc2x0 C. to about 100xc2x0 C. enables, for example, adhesion of ink to paper; (3) a UV absorber such as 2,2,6,6-tetramethyl-4-piperidinyl/xcex2,xcex2,xcex2xe2x80x2,xcex2xe2x80x2-tetramethyl-3,9-(2,4,8,10-tetraoxospiro(5,5)-undecane) diethyl]-1,2,3,4-butane tetracarboxylate; (4) an antioxidant such as antimony dialkyl phosphorodithioate; (5) a colorant, and wherein there can be generated with such inks excellent developed images on plain and coated papers with acceptable image permanence, excellent projection efficiency on transparencies without a post fusing step, and excellent crease resistance, and wherein the inks possess acceptable, and in embodiments superior lightfast, between about 90 to about 100 percent and superior waterfast between about 95 to about 100 percent values. Moreover, in embodiments of the present invention there is enabled the elimination, or minimization of undesirable paper curl since water need not be present, or minimum amounts less than about 1 percent of water may be selected in embodiments in the invention inks, and it is preferred that there be an absence of water. When water is not present in the inks, a dryer can be avoided thereby minimizing the cost of the acoustic ink jet apparatus and process.
U.S. Pat. No. 5,286,288 discloses a hot melt ink composition for use in continuous ink jet printing comprising an electrolyte, an electrolyte-solvating and dissociating compound and an image-forming agent, said ink being solid at about 25xc2x0 C., said ink liquefying at a temperature between 75xc2x0 C. and 175xc2x0 C., and said ink in the liquid stage having a conductivity of greater than about 100 microsiemens/cm 8[log(pico.mho/cm)].
The use of quaternary ammonium compounds as conductivity enhancing agents is known, reference, for example, U.S. Pat. No. 3,985,663 which discloses conductive inks containing quaternary ammonium compounds. The liquid developer has a viscosity suitable for convenient application to an imaged surface in the development of latent electrostatic images and has improved conductivity resulting from incorporating conductivity control agents in the form of quaternary ammonium compounds which are soluble in the carrier of the ink composition. The conductive ink composition can comprise a coloring agent, optionally a binder and a dispersing agent, a carrier and a quaternary ammonium compound soluble in the liquid carrier.
U.S. Pat. No. 5,382,492 discloses quaternary ammonium compounds as charge adjuvants for positive electrostatic liquid developers consisting essentially of (A) a nonpolar liquid having a Kauri-butanol value of less than 30, present in a major amount; (B) thermoplastic resin particles having dispersed therein a quaternary ammonium compound which is substantially insoluble in the nonpolar liquid; (C) a nonpolar liquid soluble ionic or zwitterionic charge director compound. The process of preparation of the electrostatic liquid developer is described. The liquid developers are useful in copying, color proofing including digital color proofing, lithographic printing plates and resists. The conductive inks of this prior art contain thermoplastic resins which can increase the viscosity of inks above, for example, 20 cps at the jetting temperatures of acoustic ink jet printing. The inks of the present invention possesses conductivity of, for example, in the range of 6.4 to about 7.0 [log(pico.mho/cm)] and are less conductive than those of U.S. Pat. No. 5,382,492 but have lower viscosities in the range of, for example, from about 5 to about 6 cps at a temperature of between about 120xc2x0 C. to about 150xc2x0 C. as required for acoustic ink jet printing.
The use of mineral acid salts or organic acid salts in aqueous polar inks are known, reference, for example, U.S. Pat. No. 5,518,534 which discloses an ink set and process for alleviating bleed in printed elements employing a first ink and a second ink, each containing an aqueous carrier medium and a colorant; the colorant in the first ink being a pigment dispersion and the second ink containing a salt of an organic acid or mineral acid having a solubility of at least 10 percent in 100 percent of water at 25xc2x0 C.
U.S. Pat. No. 5,531,818 discloses a thermal ink jet printing process which comprises (a) incorporating into a thermal ink jet printer an ink composition comprising water, a dye, and pigment particles, and chemically bonded to the surfaces thereof a hydrophilic moiety selected from the group consisting of sulfonic acid salts, phosphoric acid salts, carboxylic acid salts, and mixtures thereof, and (b) causing droplets of the ink to be ejected in an imagewise pattern onto a recording medium by selectively heating the ink in the printer in an imagewise pattern, thereby generating images on the recording medium. The aqueous inks of this prior art are, for example, 10 times more conductive than the inks of the present invention, and the inks of this prior art possess a number of disadvantages in that the ink has a tendency to soak into a plain paper medium, and this is avoided or minimized with the inks of the present invention in embodiments. This blurs the print or thins out the print locally thereby adversely affecting print quality. Problems have been encountered with thermal ink jets in attempting to rid the ink of moisture fast enough so that the ink does not soak into a plain paper medium. This is particularly true when printing with color. Therefore, usually when printing with thermal ink, one needed to use coated papers, which are more costly than plain paper.
The use of organic or inorganic salts in combination with oxyalkylene compounds, and preferably those with melting points of between about 75 to about of 120xc2x0 C. in nonaqueous, nonpolar, solid phase-change inks employed in electric field assisted acoustic ink jet printing are not believed illustrated in the above 5,531,818 patent. One advantage of a conductive phase-change ink with salts used in conjunction with electrical-field assist ink jet printing is its ability to print on plain paper with controlled ink drop placement thereby yielding low edge raggedness images. Since the phase-change ink quickly solidifies as it cools, and since it is waxy in nature, it does not normally soak into a paper medium. The inks of the prior art as disclosed in U.S. Pat. No. 5,531,818; and U.S. Pat. No. 5,518,534 contain water which means that these inks usually are not used at 150xc2x0 C. as the water will evaporate prior to jetting. The inks of the prior art as disclosed in U.S. Pat. No. 3,985,663 and U.S. Pat. No. 5,382,492 contain high boiling liquids which can possibly be used in acoustic ink jet printing but as these inks are still in the liquid form after these are jetted on to paper these smear easily and penetrate into paper yielding high degree of show through.
In acoustic ink printing, the printhead produces approximately 2.2 picoliter droplets by an acoustic energy process. The ink under these conditions should display a melt viscosity of about 5 to 6 centipoise or less at the jetting temperature. Furthermore, once the ink is jetted onto the paper, the ink image should be of excellent crease property, and should be nonsmearing waterfast, of excellent transparency and excellent fix qualities. In selecting an ink for such applications, it is desirable that the vehicle display a low melt viscosity, such as from about 1 centipoise to about 25 centipoise in the acoustic head, while also displaying solid like properties after being jetted onto paper. Since the acoustic head can tolerate a temperature up to about 180xc2x0 C., and preferably up to a temperature of from about 140xc2x0 C. to about 160xc2x0 C., the vehicle for the ink should preferably display liquid like properties such as a viscosity of 1 to about 10 centipoise at a temperature of from about 75xc2x0 C. to about 165xc2x0 C., and solidify or harden after jetting onto paper such that the ink displays a hardness value of from about 0.1 to about 0.5 millimeter utilizing a penetrometer according to the ASTM penetration method D1321.
Ink jet printing processes that employ inks that are solid at room temperature and liquid at elevated temperatures are known. For example, U.S. Pat. No. 4,490,731, the disclosure of which is totally incorporated herein by reference, discloses an apparatus for dispensing certain solid inks for printing on a substrate such as paper. The ink dye vehicle is chosen to have a melting point above room temperature so that the ink, which is melted in the apparatus, will not be subject to evaporation or spillage during periods of nonprinting. The vehicle selected possesses a low critical temperature to permit the use of the solid ink in a thermal ink jet printer. In thermal ink jet printing processes employing phase-change inks, the solid ink is melted by a heater in the printing apparatus and utilized as a liquid in a manner similar to that of conventional thermal ink jet printing. Upon contact with the printing substrate, the molten ink solidifies rapidly, enabling the dye to remain on the surface instead of being carried into the paper by capillary action, thereby attempting to enable higher print density than is generally obtained with liquid inks. Phase-change ink jets are somewhat similar to thermal ink jets, however, a phase-change ink contains no solvent. Thus, rather than being liquid at room temperature, a phase-change ink is typically a solid or phase-change having a wax-like consistency. These inks usually need to be heated, for example, to approximately 100xc2x0 C. before the ink melts and turns into a liquid. With phase-change inks, a plurality of ink jet nozzles is provided in a printhead. A piezoelectric vibrating element is located in each ink channel upstream from a nozzle so that the piezoelectric oscillations propel ink through the nozzle. After the phase-change ink is applied to the substrate, freezing on the substrate resolidifies the ink.
Each of these types of known ink jets, however, has a number of advantages and disadvantages. One advantage of thermal ink jets is their compact design for the integrated electronics section of the printhead. Thermal ink jets are disadvantageous in that the thermal ink has a tendency to soak into a plain paper medium. This blurs the print or thins out the print locally thereby adversely affecting print quality. Problems have been encountered with thermal ink jets in attempting to rid the ink of moisture fast enough so that the ink does not soak into a plain paper medium. This is particularly true when printing with color. Therefore, usually when printing with thermal ink, one needed to use coated papers, which are more expensive than plain paper.
One advantage of phase-change ink is its ability to print on plain paper since the phase-change ink quickly solidifies as it cools, and since it is waxy in nature, does not normally soak into a paper medium. However, phase-change ink jet system can be cumbersome in structure and in design, that is, the associated integrated electronics of a thermal ink jet head are considerably more compact than those of a phase-change ink jet head.
In addition, U.S. Pat. No. 4,751,528, the disclosure of which is totally incorporated herein by reference, discloses a phase-change ink jet system which includes a temperature-controlled platen provided with a heater and a thermoelectric cooler electrically connected to a heat pump and a temperature control unit for controlling the operation of the heater and the heat pump to maintain the platen temperature at a desired level. The apparatus also includes a second thermoelectric cooler to solidify phase-change ink in a selected zone more rapidly to avoid offset by a pinch roll coming in contact with the surface of the substrate to which phase-change ink has been applied. An airtight enclosure surrounding the platen is connected to a vacuum pump and has slits adjacent to the platen to hold the substrate in thermal contact with the platen.
Further, U.S. Pat. No. 4,791,439, the disclosure of which is totally incorporated by reference, discloses an apparatus for use with phase-change inks with an integrally connected ink jet head and reservoir system, the reservoir system including a highly efficient heat conducting plate inserted within an essentially nonheat conducting reservoir housing.
Ink compositions for ink jet printing are known. For example, U.S. Pat. No. 4,840,674, the disclosure of which is totally incorporated herein by reference, discloses an ink composition which comprises a major amount of water, an organic solvent selected from the group consisting of tetramethylene sulfone, 1,1,3,3-tetramethyl urea, 3-methyl sulfolane, and 1,3-dimethyl-2-imidazolidone, which solvent has permanently dissolved therein spirit soluble dyes.
U.S. Pat. No. 5,006,170 and U.S. Pat. No. 5,122,187, the disclosures of each of which are totally incorporated herein by reference, disclose ink compositions suitable for ink jet printing which comprise a colorant, a binder, and a propellant such as hydrazine, cyclic amines, ureas, carboxylic acids, sulfonic acids, aldehydes, ketones, hydrocarbons, esters, phenols, amides, imides, halocarbons, and the like. The inks of the present invention are dissimilar than the aforementioned ""179 and ""187, in that, for example, the invention vehicle selected displays acoustic loss values at a viscosity of from about 1 to about 20, and preferably 10 centipoise, when heated to a temperature of from about 125xc2x0 C. to about 165xc2x0 C., such that acoustic energy in the printhead can eject an ink droplet onto paper.
U.S. Pat. No. 5,041,161, the disclosure of which is totally incorporated herein by reference, discloses ink jet ink, which is phase-change at room temperature. The inks comprise vehicles, such as acids, aldehydes and mixtures thereof, which are phase-change at temperatures between 20xc2x0 C. and 45xc2x0 C. The ink is impulse jetted at an elevated temperature in the range of about 45xc2x0 C. to about 110xc2x0 C., at which temperature the ink has a viscosity of about 10 to 15 centipoise.
U.S. Pat. No. 4,853,036 and U.S. Pat. No. 5,124,718 disclose an ink for ink jet recording which comprises a liquid composition essentially comprising a coloring matter, a volatile solvent with a vapor pressure of 1 millimeter Hg or more at 25xc2x0 C., and a compound being solid at room temperature and having a molecular weight of 300 or more.
U.S. Pat. No. 5,667,568 discloses an ink composition comprised of a colorant and a bisamide with a viscosity of from about 1 centipoise to about 20 centipoise at a temperature of from about 125xc2x0 C. to about 185xc2x0 C.
U.S. Pat. No. 5,698,017 discloses an ink composition comprised of a colorant and a vehicle component, and which vehicle component is comprised of the condensation product of an organic acid and an amino alcohol.
U.S. Pat. No. 5,693,128 discloses an ink composition comprised of a colorant and a reversible crosslinked component vehicle obtained from the reaction product of an anhydride and an organoamine, and which ink possesses a viscosity of from about 1 centipoise to about 25 centipoise at a temperature of from about 125xc2x0 C. to about 185xc2x0 C.
U.S. Pat. No. 5,700,316 discloses an ink composition comprised of a colorant and a vehicle of a poly (oxyalkylene)-alkylate, a poly (oxyalkylene)-dialkylate, a polyoxa-alkanoate ester, or a polyoxa-alkanedioate diester, and which ink possesses a viscosity of from about 1 centipoise to about 15 centipoise at a temperature of from about 125xc2x0 C. to about 165xc2x0 C.
While the known ink compositions and processes may be suitable for their intended purposes, a need remains for acoustic phase-change conductive ink compositions suitable for electric field assisted ink jet printing. In addition, there is a need for phase-change ink compositions, which are compatible with a wide variety of plain papers and yield photographic quality images on plain and coated papers. Further, there is a need for phase-change ink compositions, which generate high quality, lightfast, and waterfast images on plain papers. There is also a need for phase-change ink jet ink compositions which generate high quality, fast-drying images on a wide variety of plain papers at low cost with high quality text and high quality graphics, and wherein the dye is retained on the paper surface while the ink vehicle can continue to spread within the paper structure. Further, there is a need for phase-change ink jet ink compositions, which exhibit minimal feathering. Additionally, there is a need for phase-change ink jet ink compositions, which exhibit minimal intercolor bleed. There is also a need for phase-change ink jet ink compositions, which exhibit excellent image permanence. Further, there is a need for phase-change ink jet ink compositions, which are suitable for use in acoustic ink jet printing processes. Additionally, there is a need for phase-change hot ink compositions suitable for ink jet printing processes wherein the substrate is heated prior to printing and is cooled to ambient temperature subsequent to printing (also known as heat and delay printing processes). There is also a need for ink compositions suitable for ink jet printing wherein high optical densities can be achieved with relatively low dye concentrations. A need also remains for ink compositions suitable for ink jet printing wherein curling of the substrate, such as paper, subsequent to printing is minimized, or avoided. Another advantage of the phase-change inks of the present invention is that the spherulite (spherical ink crystals) size during solidification can be reduced from conventional 6 to 9 micrometers to about 1 to 3 micrometers and in certain cases to 1 to 2 micrometers to improve projection efficiency and crease resistance. Moreover, there is a need for inks, especially hot melt inks containing low molecular weight polymers containing oxyalkylene segments or urea compounds with polymers containing oxyalkylene segments complexed with metal halides. These and other needs can be achievable with the inks of the present invention in embodiments thereof.